Recently, corresponding to the wide spread WDM transmission system enabling a large capacity transmission in an optical communication system, a high output of the used optical power is proceeded. The tendency of the above described optical high output (several W) is enhanced in the future.
In the fabrication field, also high output and high quality laser is required. Accordingly, an input/output resistance to the high output light (several 10 to 100 W) is required in the laser components forming inside thereof or the output component of the laser.
There has been known optical components having diversity of functions in which various optical elements are arranged between the optical fiber and the collimate lens (i.e., collimator). Furthermore, in the fabricating laser, a laser guide is connected through an input side lens portion to the output portion of the laser oscillator. The laser beam output from the laser oscillator is focused in the lens portion, then entered in the optical fiber from the incident portion, and transmitted in the core of the optical fiber. Thus transmitted beam is output from the output end of the optical fiber, then focused by the focus lens in the output side lens portion, and then irradiated.
As the optical component connected in space for a high power beam transmission used in the optical communication and machining such as welding, cutting, there are an optical isolator, a WDM filter module, a PBS (Polarization Beam splitter), a PBC (Polarization Beam Combiner), an optical circulator, a laser guide & laser output head, an optical connector and the like.
As depicted in FIG. 6, a part of the focused incident light by the collimate lens 902 is leaked in the cladding 906 by the coupling loss such as MFD mismatch or lens aberration in the end portion 904 of the optical fiber 903, and transmitted in the optical fiber 903. In the above described high power incident, the power of the leaked light in the cladding cannot be ignored. More specifically, when the leaked light in the cladding 906 is rapidly irradiated to the fiber coating portion 907 by the bending of the optical fiber 903 or the attachment of the foreign substance, the fiber coating portion is melted to cause damage.
Patent document 1 discloses that a tip portion made of sapphire is provided in the tip end portion of the sleeve, a shielding material is provided in the sleeve, or dispersal element is provided in the exposed glass portion of the optical fiber, in order to prevent the damage of the sleeve by the reflecting laser beam when the high power laser beam is output through the laser guide to the article to be fabricated from the laser oscillator, or by the incident laser beam when the optical connector is misaligned to the laser oscillator.
Patent document 2 discloses that a silica pipe is fixed by an inorganic adhesive to the surface of the cladding in the exposed glass portion of the optical fiber within the sleeve in order to prevent the damage of the jacket of the optical fiber even when the reflecting laser beam from the article to be fabricated, or the misaligned laser beam enters the cladding of the optical fiber.
In the above described optical components or fabricating laser, the optical power density becomes maximum value at each end of the optical fiber in which the focused beam by the lens is to be coupled. The loss by the optical absorption due to the dust, and the loss by the optical absorption due to the defect in the optical fiber or the dielectric multi layered filter (e.g., AR coat) are converted to heat to cause a critical defect (e.g., phenomenon such as fiber fuse), thus damaging the optical fiber, optical component or optical device or the like.
There is proposed that the diameter of the beam to be focused at the end of the optical fiber by the connecting portion of the optical connector or the lens is enlarged to lower the optical power density. Patent document 3 proposes that in order to enlarge the MFD (Mode Field Diameter) of the optical fiber, a core enlarged optical fiber is produced by the treatment in which the dopant in the core is heat-diffused (herein after referred to “TEC treatment”), or a GIF (Grated Index Fiber) is connected by fusion splicing to the end portion of the optical fiber (hereinafter referred to “GIF fusion splicing treatment”). Patent document 4 proposes that in order to enlarge the diameter of the beam to be focused on the end portion of the optical fiber, a GIF for expanding the diameter of the mode field is connected by fusion splicing to the tip end of the optical fiber and a coreless fiber having no core is connected by fusion splicing to the tip end of the GIF (herein after referred to “GIF+coreless fiber fusion splicing treatment”).
FIG. 12 is a schematic cross sectional view of the optical collimator 590 in which the tip end portion 593 of the core 592 in the optical fiber 591 is enlarged by TEC treatment.    Patent document 1: Japanese Patent Application Publication 2003-107294;    Patent document 2: Japanese Patent Application Publication 2003-139996;    Patent document 3: Japanese Patent Application Publication 2004-86127;    Patent document 4: Japanese Patent Application Publication 2005-17702;    Patent document 5: Japanese Patent Application Publication 2004-86127.